Microfilms are often prepared in a way to include frames arranged irregularly, with some original characters recorded upright and others sideways. Generally, therefore, a microfilm reader or a microfilm reader/printer includes an image rotating prism disposed between a projecting lens and a screen, i.e. at an enlargement side of the projecting lens, for correcting orientation of images for reproduction on the screen.
Since the image rotating prism is disposed at the enlargement side, a limited space is available for accommodating the projecting lens. As a result, a lens system having a short length is required.
This condition sets a great limitation to a projecting lens of about 10 magnifications having a long focal length. It sets even a greater limitation to a zoom lens system having a magnification range on the order of 10-power.
A zoom lens system allowing an image rotating prism to be disposed at the enlargement side is disclosed in U.S. Pat. No. 4,733,951, for example.
This projecting zoom lens system has a large zoom range of 6.5 to 14 magnifications. The lens system is a positive-negative, two-unit lens system comprising a first lens unit having a positive refractive power and including a field stop movable with a zooming operation and a second lens unit having a negative refractive power. This is a very compact and excellent lens system having a telephoto ratio .infin.TL.sub.L /f of 0.78 in a longest focal length condition with an object at infinity.